My case came with a mounting platform of sorts. I ended up using some 1" square steel tubing to extend the 'mounting surface' backwards, drilled holes to match up with the holes in the existing plastic rack, and bolted my case to the frame screw holes that the rack attaches to.
Because of my specific situation, I left the plastic rack in place as a spacer of sorts. The mounting hardware came with two lengths of steel, which I used to pinch the tubular steel in between, and placed some eyehook bolts with the loops at the bottom which secure the two strips of steel and thread through the mounting platform, for attaching bungies later.
Then I attached the mounting platform and case to the bike with two bolts extending through to the threaded frame bolt holes using threaded rod cut to length, two bolts that secured the tubular steel to the frame (in the middle), and the eyehook bolts holding the case at the back. Then I used wing nuts to attach the case to the platform I built, so I could remove the case at some point in the future. I also incorporated a quick release plug into the wiring for the lights on the case (expertly stuffed between the seat mount and the rack). Note that I haven't removed the case since...
Unfortunately, the bike has been pushed over/fell/tipped over/skidded multiple times since I mounted this. The case has remained in place and has not budged, despite kissing the pavement somewhat violently several times.
The side cases did not fare as well... I've ended up welding both side case antlers at least once, and one antler is currently broken again... My left case is currently being held together with gaff tape and love...
My case has quite a bit of stuff in it (oil, funnel, tool kit, jumper cables, straps, rags, etc.). When I was hanging with the local sport bike guys, I was the 'fix it' guy, so they liked having me around.
Here's a pic showing the eyehooks and the tubular steel. I wrapped the tubular steel with gaff tape, hence the rough black finish on the tubing.